Musical is hard on a poet (8)
I believe the answer is:
hamilton
'musical' is the definition.
'hard on a poet' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'on' means one lot of letters go next to another.
'poet' becomes 'milton' (John Milton).
'h'+'a'+'milton'='HAMILTON'
'is' is the link.
(Other definitions for hamilton that I've seen before include "Scottish town" , "Motorist" , "Lady . . . . . . . . was Lord Nelson's lover" , "Emma --, Nelson's lover" , "Lady mistress of Napoleon" .)